It’s another “chicken and the egg” conundrum, except this time the theme is hominid evolution. Did walking on two legs come first, and then dexterous hands? Or did the need to have hands that could manipulate tools drive the evolution of bipedality?
Paleontologists, anthropologists, and geneticists have long pondered this question. A recent study led by Atsushi Iriki from the RIKEN Brain Science Institute in Japan led the October 6 study in Philosophical Transactions of the Royal Society B that provided an answer to this problematic question about our evolution.
Iriki and his colleagues analyzed brain images from monkeys and humans in order to determine just how finger and toe functionality evolved. In monkeys and humans, toes have their own sensory area of the brain. But the key to understanding lies in the big toe. Big toes allow us to walk on two legs – they provide balance and support that animals that walk on all fours don’t have. Iriki found that humans have a separate brain region dedicated to the big toe – the use of these balance-providing appendages therefore evolved independently.
Finger dexterity in humans is also more advanced than that of monkeys. But the ability to move a hand in order to manipulate tools evolved before humans diverged from other hominids. How do we know this? A 4.4-million-year-old hominid fossil belonging to our early ancestor Ardipithecus ramidus shows manual dexterity, but this species was still a quadruped – Ardipithecus moved around archaic landscapes on all fours.
From Hashimoto et al. (2013)
Monkeys (Macaca fuscata, left) and humans (Homo sapiens, right) both have independent fingers, a sign of manual dexterity. Both species also have toes that are fused, but the human big toe is independent, whereas the monkey big toe is smaller in size compared to the other toes, and fused to the other toes.
So what does this mean? Hominids could move their fingers in intricate ways before they could walk on two feet. The authors of the study say that these two skills evolved independently. One probably didn’t drive in the other, though both resulted in the ability to walk down the street and eat an ice cream cone at the same time – and that is what makes us human.
Hashimoto, T., Ueno, K., Ogawa, A., Asamizuya, T., Suzuki, C., Cheng, K., Tanaka, M., Taoka, M., Iwamura, Y., Suwa, G., & Iriki, A. (2013). Hand before foot? Cortical somatotopy suggests manual dexterity is primitive and evolved independently of bipedalism, Philosophical Transactions of the Royal Society B, 368 (1630). doi: 10.1098/rstb.2012.0417
The Paleontological Research Institution, Ithaca, New York, is pleased to sponsor Paleontology content for This View of Life. Founded in 1932, PRI has outstanding programs in research, collections, and publications, and is a national leader in development of informal Earth science education resources for educators and the general public.